Radioactive spheroids coated with pyrolytic graphite



Feb. 28, 196? R, FINICLE 3,306,825

RADIOACTIVE SPHEROIDS COATED WITH PYROLYTIC GRAPHITE Original Filed Dec.18, 1962 INVENTOR. ROBERT L. FINICLE A T TORNEV United States PatentOfifice 3,306,825 Patented Feb. 28, 1967 3,306,825 RADIOACTIVE SPHEROIDSCOATED WITH PYROLYTIC GRAPHITE Robert L. Finicle, Rocky River, Ohio,assignor to Union Carbide Corporation, a corporation of New YorkOriginal application Dec. 18, 1962, Ser. No. 245,512, now

Patent No. 3,247,008, dated Apr. 19, 1966. Divided and this applicationDec. 28, 1965, Ser. No. 516,862

3 Claims. (Cl. 17667) This application is a division of applicationSerial No. 245,512, entitled, Process for Coating Radioactice SpheroidsWith Pyrolytic Graphite, filed December 18, 1962, now US Patent No.3,247,008.

This invention relates to a process for coating radioactive particleswith pyrolytic graphite by pyrolysis of hydrocarbons at temperatures inexcess of 1800 C., and to nuclear fuel particles having a coating ofpyrolytic graphite.

Radioactive materials, particularly uranium and uranium compounds, aresubject to certain disadvantages which must be overcome when thesematerials are used in the form of small spheroids embedded in a graphitebody, thereby constituting a nuclear fuel element. For example, nuclearfuel particles, particularly small particles, are subject to severeoxidation due to the environment in which they are used. Anotherdisadvantage is that uranium has a tendency to migrate through aconventional graphite matrix at elevated temperatures. Thesedisadvantages can be conveniently overcome by coating the particles withsome suitable substance which is relatively impermeable and resistant tohigh temperatures.

Because of its low thermal neutron cross section, relative inertness,low permeability, high sublimation point and low susceptibility toradiation damage, pyrolytic graphite is eminently suited as a coatingmaterial for nuclear fuel particles. It has been found that pyrolyticgraphite coatings deposited at relatively high temperatures areparticularly effective in minimizing the aforementioned disadvantages.However, nuclear fuel particles cannot be heated directly totemperatures which are high enough to provide optimum coatingcharacteristics due to the tendency of the particles to fuse tothemselves and to the container walls at such temperatures.

Therefore, it is an object of this invention to rovide a process forcoating nuclear fuel particles with pyrolytic graphite at temperaturesabove the surface sintering of the radioactive particles whereinadhesion of the particles to themselves and to the coating chamber wallis avoided. It is another object to provide the particles with a highdensity, high strength barrier with which to retain the gaseous fissionproducts of these radioactive materials. It is a further object tominimize undesirable oxidation of the fuel particles. A still furtherobject is to substantially reduce migration of uranium through thegraphite matrix of the fuel element at elevated temperatures.

These and other related objects are achieved by de- This process whichresults in a dual layered coating of pyrolytic graphite is applicable toparticles of uranium carbide which have a surface sintering temperaturebelow 1800 C. as well as to materials having a surface sinteringtemperature above 1800 C., e.g., thorium carbide.

The term surface sintering as used herein, including the appendedclaims, represents the temperature at which the surface of the particlescoalesce, fuse or otherwise bond together or to the coating chamberwalls. The surface sintering temperature is dependent, at least in part,on the nature of the particulate radioactive material and on the size ofthe particles.

The laminar layer of pyrolytic graphite is characterized as not having acrystalline structure while the columnar pyrolytic graphite has adistinct crystalline structure visible through the use of polarizedlight.

This process is intended to provide a thick outer coating of columnarpyrolytic graphite having a density of at least 1.8 grams per cubiccentimeter.

In the practice of the instant invention pyrolytic graphite is depositedon radioactive particles by placing the particles in a suitablecontainer and heating them to a temperature slightly below the surfacesintering point of the radioactive particles while surrounding theparticles with an atmosphere composed of an inert gas and a gaseoushydrocarbon. The elevated temperature causes the hydrocarbon gas tocrack or pyrolyze thus depositing free carbon atoms on the surface ofthe radioactive particles.

The particulate radioactive material is preferably kept in motion whilethe pyrolytic graphite is being deposited by some suitable means such asa rotating capsule, a fluidized bed or a vibrating surface. This insuresthat the entire surface of each particle is exposed to the hydrocarbongas and thus provides for a uniform coating of pyrolytic graphite. Oncethe initial thin layer of pyrolytic graphite has been deposited thetemperature is raised above the surface sintering temperature of theradioactive particles and the deposition continued until a coating ofthe desired thickness is built up. The final coating can be applied attemperatures ranging from about 1800 C. to about 2200 C.

The coating process is preferably interrupted after the deposition ofthe initial thin layer of graphite, and the particles are then removedfrom the coating chamber and washed with an aqueous solution of a strongacid such as nitric acid, hydrochloric acid, sulfuric acid, and thelike, at an elevated temperature, preferably from C. to about 98 C. Thisserves to eliminate any surface contamination and also to dissolve anyparticles which may be uncoated and therefore would tend to sinter athigh temperatures. The cleaned particles are then replaced in thecoating chamber and heated to the desired temperature and the finalouter coat of pyrolytic graphite deposited.

The rate of deposition of both the inner and outer layer can beconveniently regulated by adjusting the concentration of the hydrocarbongas in the container and the rate at which the gases, i.e., thehydrocarbon gas and the inert gas, flow through the air tight container.Preferably the gas mixture is composed predominantly of an inert gaseousdiluent such as argon, helium or hydrogen. The temperature at which thedeposition is carried out is also a factor which influences the rate ofdeposition. Finally the period of time for which the low temperaturedeposition is maintained may be varied in order to control the thicknessof the several layers of pyrolytic graphite.

Any hydrocarbon which will pyrolyze to provide free carbon atoms at atemperature below the surface sintering point of the radioactiveparticles can be employed for the deposition of the initial layer ofpyrolytic graphite. The same or a different hydrocarbon may be used forthe high temperature deposition of the outer coating. Suitablehydrocarbons include aromatic hydrocarbons such as benzene; aliphatichydrocarbons such as the alkanes, e.g., methane, ethane, propane,butane, and the like; the alkenes, e.g., ethylene, propene, butene,pentene and the like, alkylenes, e.g., acetylene; cycloaliphatichydrocarbons such as cyclobutane, cyclopentane, cyclohexane,cyclobutene, cyclopentene can also be used. Preferred hydrocarbons arethe low alkanes having up to 10 carbon atoms inclusive.

For the deposition of the initial thin layer of pyrolytic graphite theinert gaseous diluent is mixed with the hydrocarbon gas in a volume tovolume ratio ranging from about 3 parts of diluent to 1 part ofhydrocarbon to about 20 parts of diluent to 1 part of hydrocarbon.

For the deposition of the final outer coating of pyrolytic graphite thevolume ratio of diluent to hydrocarbon is preferably maintained fromabout 7 to l to about 20 to 1. Highly satisfactory results have beenobtained by using a volume ratio of 10 parts of diluent per part ofhydrocarbon during the deposition of the outer coating of pyrolyticgraphite.

Relatively high ratios of diluent to hydrocarbon are generally preferredsince these tend to produce less soot in the container, facilitatecontrol over the thickness of the deposit and provide more efficientutilization of the hydrocarbon gas.

The initial deposit of pyrolytic graphite must be of sufficientthickness to prevent fusion of the radioactive particles during the hightemperature deposition of the outer layer. Suitable initial layers canbe from about microns to about 50 microns or higher in thickness.Preferably the initial layer is from about 5 to about 20 microns thick.

The thickness of the final outer layer of pyrolytic graphite is notnarrowly critical and will depend in large measure on the end use forwhich the coated radioactive substance is prepared.

As was previously mentioned, the temperature at which the initial layerof pyrolytic graphite is deposited must be lower than the surfacesintering point of the radioactive particles. Therefore, the temperatureof the initial deposition must be determined in view of the particularradioactive material to be coated. The outer layer of pyrolytic graphiteis deposited at temperatures above the surface sintering point of theparticulate radioactive material and, in any event, above 1800 C. Themaximum obtainable deposited density in pyrolytic graphite is reached atabout 220 C. In order to minimize any tendency toward migration of theuranium, deposition temperatures above 2200" C. are not recommended. Theactual temperature at which the final coating is applied should beselected upon consideration of the temperatures to which the finalproduct will be exposed.

The coefficient of thermal expansion of nuclear fuel materials aregenerally greater than the coefficient of thermal expansion of pyrolyticgraphite. Because of this differential there are advantages todepositing the pyrolytic graphite at as high a temperature as possiblewithin the aforesaid limitations. When the coated radioactive particlecools the particle shrinks within the graphite shell thus leaving aspace for reexpansion upon any subsequent heating and thereforeminimizing the danger of cracking the protective shell of pyrolyticgraphite due to the internal stress caused by the expansion of theradioactive particle.

While the present invention is applicable to any radioactive metal ormetal compound it is particularly applicable to coating radioactivemetal carbides with pyrolytic graphite.

The figure is a photomicrograph showing a sectional view of a uraniumcarbide particle 1 coated with an inner layer of laminar pyrolyticgraphite 2 and an outer layer of columnar pyrolytic graphite 3.

The following examples will further illustrate the present invention.

Example I Fifty grams of uranium dicarbide particles having a sinteringtemperature of 1775 C. and ranging in size from 177 to 250 microns wereheated in a fluidized bed furnace having an inside diameter of 1 inch.The particles were suspended by helium flowing at a rate of 4.3 litersper minute. When the temperature of the particles was between 1700 C.and 1750 C. methane was introduced at a rate of 1.2 liters per minute.These conditions of temperature and flow rate were maintained for 15minutes after which the methane flow was shut off and the particlesallowed to cool. The uranium carbide particles were found to have acoating of laminar pyrolytic graphite which was from 12 to 14 micronsthick.

The coated particles were then removed from the furnace and washed in an8 molar solution of nitric acid to eliminate impurities and surfacecontamination. Then 15 grams of the coated particles were replaced inthe fluidized bed furnace and fluidized by helium flowing at the rate of4.5 liters per minute. The particles were heated to 1800 C. at whichpoint methane was introduced at the rate of 0.6 liter per minute. Theseconditions were maintained for a period of 1 hour and 45 minutes duringwhich a layer of columnar pyrolytic graphite 60 to microns thick wasdeposited.

Example ll Thirty grams of uranium dicarbide particles ranging in sizefrom about 50 to about microns and having a surface sinteringtemperature of about 1550 C. were heated in a fluidized bed furnacehaving an inside diameter of 1 inch. The particles were suspended byhelium flowing at a rate of 3.5 liters per minute. When the temperatureof the particles was between about 1450 C. and 1500 C. methane wasintroduced at a rate of 1.0 liter per minute. These conditions oftemperature and flow rate weremaintained for a period of about 20minutes after which the methane flow was shut off and the particlesallowed to cool. The particles were found to have a coating of laminarpyrolytic graphite which was approximately 10 microns thick.

The coated particles were washed in 8 molar nitric acid and thenreplaced in the furnace. The particles were fluidized by helium flowingat a rate of about 4.5 liters per minute and heated to about 1800 C., atwhich point methane was introduced at a rate of about 0.6 liter perminute. These conditions were maintained for a period of about 1 hourand 45 minutes during which period a layer of columnar pyrolyticgraphite having a thickness of about 60 to 80 microns was deposited.

What is claimed is:

1. A radioactive particle having an inner uncracked coating of laminarpyrolytic graphite and an outer coating of columnar pyrolytic graphite,the interior volume enclosed by said inner coating being greater thanthe volume of the radioactive particle at all temperatures below thesurface sintering temperature of the particle.

2. A product as in claim 1 wherein the radioactive particle is uraniumcarbide.

3. A product as in claim 1 wherein the radioactive particle is thoriumcarbide.

(References on following page) References Cited by the Examiner UNITEDSTATES PATENTS Reactor Core Materials, v01. 4, No. 2, May 1961, pp.58-59.

References Cited by the Applicant Sawman et a1. 17691 X H Johnson et 1176 67 X r AEC Report NYO 9064, Dec. 6, 1961, pp. 5-1 through Goeddel 1791 X 57, 5-12, 515 and 15-16.

Huddle 17691 X CARL D. QUARFORTH, Primary Examiner.

OTHER REFERENCES AEC Report BMI 1489, Mar. 10, 1961, Dayton st 211., pp.L-l to L-S.

BENJAMIN R. PADGETT, Examiner.

10 M. I. SCOLNICK, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No- DatedFebruary 28,

Inventor(s) Robert L. Finic 1e It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 3 line 57 "220C" should read --2200C-- Signed and Scaled thissixteenth Day Of September 1975 [SEAL] A ttesr:

RUTH C. MASON C. MARSHALL DANN Arresting ()jfmr (umrm'ssium-r uj'larenrsand Trademarks

1. A RADIOACTIVE PARTICLE HAVING AN INNER UNCRACKED COATING OF LAMINARPYROLYTIC GRAPHITE AND AN OUTER COATING OF COLUMNAR PYROLYTIC GRAPHITE,THE INTERIOR VOLUME ENCLOSED BY SAID INNER COATING BEING GREATER THANTHE VOL-